[Influence of technical factors in the determination of minimal inhibitory concentration by microdilution]

Minimal inhibitory concentrations (MICs) of a new quinolone, norfloxacin, as determined by agar dilution are greater than those found using a liquid dilution micromethod. We report herein an analysis of the various parameters possibly involved in this discrepancy: Mueller-Hinton and BioMérieux, glass or plastic, small or large inoculum, and comparative volumes in which the inoculum (IN) and antibiotic (AB) are presented (1 ml IN + 1 ml AB or 1.5 microliter IN + 50 microliter AB or 50 microliter IN + 50 microliter AB). Volume of the inoculum suspension had a bearing on the results obtained with all three reference strains tested. Norfloxacin MICs for S. aureus 7625 and P. aeruginosa 76110 increased commensurately with the ratio of inoculum volume to antibiotic volume, and vice versa. In contrast, no significant variation was found for E. coli 7624. To evaluate the frequency of this effect, we tested 40 antibiotics on the reference strains, and several antibiotics on savage strains (16 Enterobacteriaceae, 40 Staphylococcus, 12 Pseudomonas and 17 P. aeruginosa). The significance of inoculum and antibiotic volumes was corroborated for some antibiotics. Results most consistent with the reference method were obtained with 50 microliter inoculum and 50 microliter antibiotic solution.